Coming as it did a few months after the formal launch of the Jawaharlal Nehru National Solar Mission (JNNSM) in January 2010, the mandate of The National Centre for Photovoltaic Research and Education (NCPRE) was to provide the research, education, and training backdrop required for achieving the JNNSM targets. Given the highly crossdisciplinary nature of photovoltaics (PV), it was felt that NCPRE should be a broad-based centre, leveraging IITB’s excellent track record of collaboration between departments and goal-oriented group activity. A team of faculty members from several departments got together, and, together with students, have been working on several major aspects of photovoltaics—solar cells, materials, power electronics, storage, deployment, module reliability, modelling, and simulation. At present, 30 faculty members from eight departments participate in NCPRE, together with 120 post-graduate students and research assistants, ably supported by several administrative and technical staff members. NCPRE has progressed well since its inception in 2010. This article provides an overview of the activities and achievements of NCPRE over the last five years.

Creation of Facilities

Through the funding received from MNRE, as well as inputs from IITB, several new laboratories and facilities specific to photovoltaics have been created. Equipment worth about `23 crore has been purchased, deployed, and used. The facilities and laboratories have been widely used not only by NCPRE faculty and students, but also by other faculty and students from within and outside IIT (26 other institutions), as well as 14 PV industries. The funding from MNRE has thus created a major ‘resource centre’ in terms of facilities which are available for use by researchers from NCPRE as well as outside. Pictures 1 (a) to (d) show some of the laboratories created by NCPRE.

Education and Training

Education has been an important part of the activities of NCPRE, as the success of JNNSM would require trained manpower. NCPRE has contributed to education and training in a variety of ways: offering PV courses at IITB for UG and PG students; enhancing the number of MTech and PhD students in PV at IITB; running the ‘Teach a 1,000 Teachers’ programme to train teachers from other colleges; developing a lowcost PV lab kit and distributing 200 of them; developing a low-cost students’ fab-lab, suitable for replication at other universities; running many short-term courses for industry personnel, teachers as well as for master trainers for technicians. A lab manual titled, Solar Photovoltaics: A Lab Training Manual and a technician training manual titled, Solar Photovoltaic Technology and Systems: A Manual for Technicians, Trainers, and Engineers have also been published. Some details are given in Table 1, and the low-cost lab kit is shown in Picture 2.

Silicon Solar Cells

The mainstream solar cell technology continues to be silicon, and therefore NCPRE’s involvement in this area was deemed to be important, especially as it would allow interaction with the industry in India. The target was to achieve 18 per cent efficiency over a 100 cm2 area. Work on this at NCPRE started in August 2013 with the commissioning of the ‘NCPRE Clean Room’ and installation of equipment. All unit processes were developed for fabrication of solar cells on industry-size silicon wafers of size 5” X 5” (Figure 1a) and 6” X 6”. The cell architecture developed is the screen-printed contact, aluminium back surface field structure, which is widely used by the industry. A stack of low temperature thermal oxide and PECVD SiNX:H is used for the front surface passivation. The best cell efficiency achieved so far is 17.84 per cent on 155 cm2 p-type mono-crystalline silicon wafers (Figure 1b). The evolution of the cell efficiency is shown in Figure 1 (c). During the two years of the development of the cell process, the silicon cell research team at NCPRE, which includes a large number of students, has gained significant expertise in various unit processes, process integration, characterization techniques, and loss analysis methods. The team of NCPRE members has also developed wafer level characterization set-ups for electroluminescence (EL) and photoluminescence (PL). The cells fabricated at NCPRE have been extensively characterized using EL, PL, I-V, QE, spectrophotometry, contact resistance scan, lifetime, etc., and for developing models for 2D simulations. Using the cell baseline process as a platform, the team is currently engaged with several industries for: silver paste development; demonstration of double print contact process; and analysis and design of front contact grid.

Several novel processes are being developed by PhD students working in the project. Examples include: Nickel (Ni)/Copper (Cu) front metallization has been done by electroless plating of Ni and electroplating of Copper; a PERC cell process with SiNX:H backsurface passivation, laser fired Al back contacts and Ni/Cu front contacts has been demonstrated with an efficiency of 15.5 per cent; dielectric-metaldielectric plasmonic structures have been developed for reducing front surface reflections (an improvement in JSC of 1.2 mA/cm2 is demonstrated); a low temperature 350°C plasma oxidation process has been developed, and the layer integrated in our cell process and compared with thermal oxide; a spray coating process for Al2O3 is being developed and a surface recombination velocity of 28 cm/s demonstrated on p-type silicon surface.

New Materials and Devices

One of the major activities taken up at NCPRE under ‘New Materials and Devices’ was the development of >5 per cent semiconductor-sensitized solar cells, where the dye of a dyesensitized solar cell (DSSC) is replaced by a thin semiconductor layer, usually on a mesostructured scaffold. As a part of the NCPRE activity, the team of NCPRE members has developed and set up an atomic layer deposition (ALD) system (Picture 1c) for depositing various extremely thin absorber (ETA) layers on mesoporous layers. The system was successfully employed to deposit conformal thin films of various semiconducting absorber layers on porous oxide structures and blocking interfacial layer at TCO/porous-TiO2 interface in ETA configuration. A detailed study on preventing the recombination losses by surface passivation using ultra-thin ALD grown metal oxides at the absorber/transporter interface has been performed. Amorphous Sb2S3 as an absorber material yielded a cell with an open circuit voltage of 380–400 mV and a short circuit current of approximately 1.5 mA/cm2. Initial studies on annealed crystalline Sb2S3 based ETA cells have resulted in a Voc of 500 mV and a Jsc of approx. 9.5 mA/cm2 with an overall efficiency of approx. 2 per cent. Subsequently, in an effort to minimize the back recombination in the sensitized mesoporous structures, the team also employed organic-inorganic lead halides (perovskites) as sensitizing material in the meso-structured scaffold based on titania. With optimization of the device structure, approx. 8.9 per cent efficient devices were obtained, as shown in Figure 2. The devices were fabricated in a complete solid state configuration with spiro-OMeTAD as hole transport layer.

As is now known, the perovskite MAPbI3-xClx (methyl ammonium lead iodide-chloride mixture) is ambipolar in nature with large diffusion lengths and a high absorption coefficient. This allows the removal of mesoporous scaffold layer, resulting in a simple planar bulk heterojunction device. Thermal co-evaporation or vapourassisted vacuum deposition was employed by the team to deposit the perovskite absorber layer, and planar cells with efficiencies as high as 14 per cent have been obtained, as shown in Figure 3.

In addition to the work on SSSC and perovskites, NCPRE members have also worked on OPV, conventional DSSCs and QD based devices. In OPV devices, a comparative study was conducted on polymer solar cells (PSC) fabricated with defect-free poly (3-hexylthiophene) (DF-P3HT) and Region-Regular (RR) P3HT. A maximum efficiency of 3 per cent and 4.1 per cent were achieved for RR and DF P3HT devices, respectively with a fill factor of 68 per cent in the DF-P3HT case, which is one of the highest reported values. Transient photocurrent and photovoltage measurements were done on these devices to estimate the decay time constant (τ), charge carrier density (n), and bimolecular recombination coefficient (k). In DSSCs, work on TiO2 nanorod-based devices has been carried out. Work has also been conducted on QD-based wavelength shifting for PV devices, and QD-based all-Si tandem cells.

Power Electronics and PV Systems

Besides the work on solar cells (both silicon and other materials), NCPRE has done a great deal of research activity on power evacuation strategies through power electronics interfaces and PV microgrids, as well as on storage aspects related to PV systems. The motivating theme of the power electronics activity has been to devise strategies and circuits which are robust, reliable, and efficient, and therefore, which can be deployed in the rural sector of India. Accordingly, the following systems have been developed: (i) Development of a reliable and efficient stand-alone 500 VA solar PV system for rural household applications (Picture 3a). While developing these schemes the main consideration was to reduce the number of intermediate power conversion stages, which led to the increase in efficiency and also the reliability of the system due to the reduction in component count. Four different topologies have been developed. Operating full load efficiencies of the topologies are approx. 90 per cent; (ii) Development of solar PV pumping systems. In the agricultural sector, induction motor driven irrigation pumps are the main work horse, so a power electronics interface along with a PV module to feed the already deployed induction motors would optimize resources. A drive has been developed wherein the front-end converter is an interleaved DC to DC converter to reduce the size of the magnetic elements. It is known, however, that Brushless DC (BLDC) motor-based pumps are more efficient than conventional induction motors. Hence, for new installations, a low-cost tubular BLDC motor (using ferrite magnets instead of rare earth magnets), shown in Picture 3b, has been designed and fabricated having an estimated efficiency of approx. 90 per cent; (iii) A reliable 5 kVA transformer-less rooftop grid connected system has been developed (Picture 3c), using a multilevel inverter topology so that the size of the magnetic filtering devices is reduced considerably. The switching devices of the converters are SiC devices and the part load efficiency of the overall system is found to be 94 per cent.

In addition to work on power electronics, NCPRE has also worked on solar PV microgrids in the following areas: novel techniques for analysis of sources in microgrids; novel inertia design methods for islanded AC microgrids with static and rotating energy sources; new strategies for energy management in microgrids; and new techniques for transient response improvement in microgrids during islanding.

In the area of storage, NCPRE has concentrated on the development of Li-ion battery storage suitable for PV applications. The activities include: development of low-cost (LiMn2O4) and high energy density (LiMn1.5Ni0.5O4) cathode systems for Li-ion batteries; development of high-performance and high tap density LiFePO4 cathode; novel anode systems for Li-ion and Na-ion battery systems. This work has resulted in several patents, and we now have the technology to make industry grade Lithium-ion battery with 0.75 Ah pouch cell. We have started making such battery cells and have been testing them in solar lantern applications.

Characterization, Modelling, and Module Reliability

Characterization plays an important part in PV, from the characterization of new materials to cells to modules. NCPRE has set up excellent characterization facilities for all these measurements, which are widely used. In addition, techniques like electroluminescence (EL) and photoluminescence (PL) have been developed in-house (Picture 4a), and new variants developed. A new portable EL image processing system and technique (Picture 4b) has been developed which allows module images to be taken in the field in daylight conditions (normally modules would have to be taken to a dark room lab). EL images taken in a dark room and in the field in daylight are shown in Pictures 4 (c) and (d), indicating excellent match.

Characterization of module reliability and degradation in India has been taken up in a major way by NCPRE (together with NISE). The NCPRE team conducted two ‘AllIndia PV Module Surveys’ in 2013 and 2014, where modules at various sites covering all climatic zones and different technologies were taken up. The 2013 as well as 2014 data have generated many interesting results. One of the concerns arising from this field data is the higher than expected degradation rate (% reduction in Pmax per year) in the hot climates of India. In addition to the All-India surveys, continuous monitoring of PV modules of different technologies has been going on for the last three years on the NCPRE rooftop to assess relative performance and degradation.

Deployment and Policy

NCPRE has been involved in several deployment-related activities and surveys. At the request of KSEB, the NCPRE team undertook a survey of rooftop PV potential in semi-rural Chendamangalam panchayat in Kerala, performing an exhaustive survey of all buildings and electricity usage patterns in the panchayat, covering 6,500 structures including residences, shops, institutions, and public buildings. The survey found that there is a potential of 11 MW of rooftop PV which can supply annually about 12 million units of electricity as against the panchayat’s requirement of 6.8 million units. A unique study in Thiruvananthapuram to utilize rooftop PV for shifting the peak load was carried out by NCPRE, and launched by KSEB as the ‘SunShift’ programme. NCPRE also helped design the hardware system for this programme. A survey of rooftop potential in Mumbai city has been undertaken, by evolving a comprehensive methodology which combines GISbased imaging, together with some site visits to calibrate the GIS data, and estimation of shadowing effects from nearby structures. This work was done by NCPRE jointly with Centre for Urban Science and Engineering (C-USE, IITB), Bridge to India (BTI), Observer Research Foundation (ORF) and IEEE Bombay Section, and has yielded a preliminary rooftop PV potential of about 1.5 GW for Mumbai.

Industry Affiliate Programme and Outreach

At the initiation of NCPRE, it was decided that in order for its work to be relevant to the solar PV industry in India, it should engage strongly with industry. Accordingly, an ‘Industry Affiliate Programme’ was launched in 2011, which currently has 18 industry/ NGO members.

In order to ensure good outreach and dissemination of information, NCPRE launched its website www. ncpre.iitb.ac.in in 2011. NCPRE has also participated in exhibitions including Intersolar India (4 times) and Solarcon (2 times), where its booths have attracted a lot of attention.

NCPRE as a PV Resource Centre

Through the work carried out during the last five years in various aspects of photovoltaics, NCPRE has emerged as a major ‘PV Resource Centre’ for India. NCPRE has a critical mass of faculty, students, and research staff, coming from different backgrounds. The critical mass and diversity of expertise together ensure that any problem in PV can be adequately addressed. Arising out of the establishment of NCPRE, the PV group at IIT Bombay has got engaged in a number of other PV-related activities in India and internationally. These include: a major involvement in the Solar Energy Research Institute for India and the US (SERIIUS); the launching of the SOUL programme for a unique methodology for the distribution of 1 million solar lamps for school children; and the launch of a start-up company in IITB’s technology incubator.

In conclusion, NCPRE has made significant progress since it was established in 2010, and has emerged as a major centre for PV education and research in India, and one of the leading centres in the world. The funding and continued support in many other ways from MNRE, its expert committees, and the NCPRE Advisory Committee are gratefully acknowledged in achieving this status.

NCPRE Research Team, Email: ncpre@iitb.ac.in